This invention relates to EPDM rubber, cross-linked in more than one way, and to compositions containing such rubber together with other rubbers and rubber compounding ingredients.
EPDM rubber, a terpolymer from ethylene, propylene and a minor amount of a non-conjugated diene monomer, can be cross-linked in a variety of ways. Since it contains some olefinic unsatuation, it is susceptible to sulfur-vulcanization, by using organosulfur accelerators. Phenolic resin-type curatives are also used to cross-link EPDM rubber, and organic peroxides, which decompose at elevated temperatures, are also effective cross-linkers for EPDM rubber.
Caywood, in U.S. Pat. No. 4,010,223, describes adducts of EPDM rubber, which adducts are formed by the reaction of, for example, maleic anhydride with the rubber at an elevated temperature. The use of a free radical generator is avoided so that the maleic anhydride does not cross-link the rubber, but, it is theorized that the maleic anhydride adds to the polymer chains so as to attach pendent succinic anhydride groups thereto. Subsequent reaction of the maleic anhydride-treated EPDM with metal salt "curing agents," such as zinc acetate dihydrate creates ionic linkages which are thermally labile so that the "cured" adducts are thermoplastic and can be shaped and further compounded on a warm rubber mill. Although the adducts can also be cross-linked with diamine compounds, the patent notes that this latter type of cross-linking is "permanent," in that the rubber is then no longer thermoplastic.
Giller U.S. Pat. No. 3,287,440 describes the cross-linking of EPDM rubber by the use of "phenol aldehyde resins," such as methylol phenolic resins, in the presence of a halogen donor and a heavy metal compound. The cross-links formed by this method consist of one or more resin units which connect two polymer chains; thus a network is formed.
Sulfur vulcanization of EPDM rubber forms mostly polysulfidic cross-links which connect polymer chains, as is well documented in the literature.
Cross-linking of EPDM rubber by using organic peroxides results in the formation of networks in which the polymer chains are joined by carbon atom to carbon atom linkages.
Of the cross-links for EPDM rubber discussed above, those obtained by using the metal-salt "curing agents" described by Caywood can be described as ion-cluster cross-links or ionomeric cross-links. The remaining types of cross-link can be termed "covalent cross-links."